The present invention relates to an improved bonding method and apparatus and particularly to such a method and apparatus for fusing two adjoining plastic surfaces.
Plastic materials have found widely varying applications both as basic structural elements and as accessory elements such as coverings or enclosures of a rigid or a flexible plastic for packaging of other elements and the like. Thermoplastic materials of similar as well as many that have dissimilar characteristics can be permanently connected through the heating of the interface to a critical temperature and applying an appropriate pressure with the heat at the critical temperature. A particularly satisfactory method and system is shown in U.S. Pat. No. 3,574,031 to Heller et al. As more fully disclosed therein, discrete susceptor particles are introduced into the fusion area and subjected to a high-frequency magnetic induction field, generally in the radio-frequency range. The plastic material immediately adjacent to the particles is converted to a fluid state, either a liquid or at least a softened flowable state, as a result of the heat generated in the particles as such. By the proper control of various parameters, the flowing condition of the plastic is accurately controlled and produces a highly satisfactory fusion bond or weld at the interface of the surfaces. As further disclosed therein, the susceptor particles may be disposed directly upon the surfaces to be joined or supported within an intermediate bond agent or layer interposed between two opposed substrates. The particles may also be embedded directly within one or both of the surfaces to be joined. As particularly disclosed in the above patent and others held by present assignee, the use of the discrete particles does produce a highly satisfactory and novel method of heating and of sealing thermoplastic members. However, as also pointed out in such art, the creation of an effective and firm bond between plastic materials requires consideration of many parameters including control of the proper temperature, proper pressures and, in particular, control of the heat location. Further, the joining of certain different particular plastics is extremely difficult because of the different sealing characteristics -- thus, a particular optimum heat, pressure and time for the plastic of the one element may not be the same for the plastic of the second element. Although an intermediate bonding agent may contribute to an improved bond because of characteristics more compatible with the two different plastics, the results have not been considered highly satisfactory in many cases.